1. Field of the Invention
The present invention generally relates to semiconductor substrates and semiconductor devices using the same. More particularly, the present invention relates to a semiconductor substrate having a gallium nitride (GaN) layer formed on a silicon (Si) substrate, and a semiconductor device such as a semiconductor substrate.
2. Description of the Related Art
Semiconductor devices using GaN are used as power devices operating at high frequencies and outputting high power, light-emitting diodes and laser diodes emitting lights of short wavelengths. Among these semiconductor devices, particularly, there has been considerable activity in the development of semiconductor devices capable of amplifying microwaves, quasi-millimeter waves or millimeter waves, such as high electron mobility transistors (HEMT), and light-emitting devices such as laser diodes (LD), light-emitting diodes and vertical cavity surface emitting laser diodes (VCSEL).
Generally, a sapphire substrate or silicon carbide (SiC) is used as a substrate for growing a GaN layer. Since the sapphire substrate and SiC substrate are expensive, an alternative technique for growing the GaN layer on a Si substrate has been developed. Silicon and gallium react easily. Thus, an aluminum nitride (AlN) layer is provided as a barrier layer between the Si substrate and the GaN layer. However, the GaN layer having good crystallinity cannot be obtained when the GaN layer is grown directly on the AlN layer. Japanese Patent Application Publication No. 2004-524250 discloses a technique taking the above into consideration in which an AlGaN layer is provided between the AlN layer on the Si substrate and the GaN layer and the Al composition continuously changes. Japanese Patent Application Publication No. 2000-277441 discloses a technique in which an AlGaN layer having a stepwise changing composition is provided between the AlN layer on the silicon substrate and the GaN layer.
However, the techniques disclosed in the above publications do not realize the GaN layer that is grown on the Si substrate and has good crystallinity. For example, a measurement result shows that the X-ray diffraction rocking curve full width at half-maximums (XRC-FWHM) of the (002) or (102) plane of a GaN layer formed on a Si substrate using the conventional technique ranges from 600 seconds to 800 seconds. Further, the substrate having the GaN layer grown on the Si substrate may have a crack on the surface or a warp due to the difference in thermal expansion coefficient between the GaN layer and the Si substrate.